Supergirl battles a scientist who has been possessed by an alien parasite, giving the ability to drain the lifeforce from anyone he touches. Meanwhile, Alex comes out to Kara, and the Guardian makes his first public appearance.

When the Danvers celebrate Thanksgiving, Alex tries to find the right time to tell her mother Eliza that she's gay. Meanwhile, Lena is forced to choose when Supergirl tells her that Lillian plans to unleash a virus that will wipe out every alien in the city.

Crossover among four CW shows: Supergirl "Medusa"; The Flash "Invasion!"; Arrow ‘Invasion!"; DC's Legends of Tomorrow "Invasion!"